Eukaryotes Genomes - DICTYOSTELIUM DISCOIDEUM

Dictyostelium discoideum are a free living amoebae. It is a cellular slime mold whose natural habitat is the upper layer of soil in deciduous forests. There, the decaying organic material allows the growth of an abundant population of telluric (land inhabiting) micro-organisms (bacteria) on which these protozoans feed. When the food supply is abundant, Dictyostelium discoideum are loners, they are content to crawl through their environment, absorbing whatever food they come across. However when the food supply is exhausted they undergo a complex developmental cycle in which about 10.000-50.000 individual amoebae aggregate to form a multicellular fruiting body. They exchange chemical signals with their freely moving neighbours and using their protruding pseudopoda (false legs) for propulsion, they seek each other out and stick together to create a multicelled creature that can sense both light and heat and can travel faster and farther than any of its component cells travelling alone. The aggregated cells then differentiate into two distinct types, stalks and armored spores - then the starving Dictyostelium is ready to sow its dormant spores into the winds, seeking a more hospitable environment.

The aggregation of individual Dictyostelium amoebae occurs by chemotaxis to periodic cAMP signals. Upon starvation a few amoebae acting as the aggregation centre start to produce periodic cAMP pulses, which are detected, amplified and relayed by the surrounding amoebae. Individual cells move chemotactically towards increasing cAMP concentrations. cAMP waves are visible as expanding spirals or concentric rings. Later the amoebae form aggregation streams and finally collect in a hemispherical aggregate, the mound. There the amoebae differentiate into two distinct cell types - the prestalk and prespore cells - which later will form the stalk and spore head. The prestalk cells locate in the tip on top of the mound, which elongates, topples over and forms the slug. In the slug 105 indivdual amoebae behave as a single organsim. The slug stage enables cellular slime moulds to migrate towards the surface of the litter stratum thus to have a better chance for spore dispersal.

D. discoideum is a useful system for the study of biological pattern formation and cellular communication. The 34 Mb genome contains many genes that are homologous to those in higher eukaryotes and are missing in Saccharomyces cerevisiae. The organism is uniquely suited for studies of cytokinesis, motility, phagocytosis, chemotaxis, signal transduction, and aspects of development. Many of these processes, which play important roles in health and disease, are either absent or are less accessible in other model organisms. In addition, having both single cell and multicellular life stages offers an excellent system for research in developmental and cell biology. The simplicity of the life cycle facilitates mutant selection, where the distinct growth and development stages are easily switched using nutrient variation.

Researchers are now interested in whether the metamorphosis from the one-celled state of Dictyostelium to its multicelled state could have something in common with the highly structured flocking of birds, schooling of fish, the collective movement of ants and even the movements of human pedestrian and vehicular traffic.

References:

hthttp://dictybase.org/
http://www.zi.biologie.uni-muenchen.de/zoologie/dicty/dicty.html
http://www.sanger.ac.uk/Projects/D_discoideum/
http://fig.cox.miami.edu/~bhoward/bil150_99/amoeba.html
http://www.ncbi.nlm.nih.gov/genome/guide/dicty/